The invention relates to a new method and mechanism for cooling a gas discharge tube on an atomic emission detector. More specifically, the instant invention relates to a method and mechanism for cooling a discharge tube employed in an atomic emission detector that is cooled using air as the coolant.
One type of detector used in chromatographic systems is the atomic emission detector, alternatively referred to as a gas discharge detector. Atomic emission detectors employ the use of a gas discharge tube oriented to receive the effluents passing out of a chromatography column. The effluents entering the discharge tube are energized into excited atoms or molecules. As the electrons of the excited atoms or molecules return to their ground state, a characteristic light is emitted that is unique to an element or molecular bond. In a gas discharge detector, the light emitted as effluents enter the discharge tube is detected and can be plotted as a function of time.
A plasma generator, such as those powered by radio frequency (RF) or microwave energy, is utilized to provide energy to cause a plasma discharge to be formed in the discharge tube. In a typical application, energy is confined in a discharge tube whose dimensions are on the order of 1 mm diameter by 5 to 10 mm in length. This large amount of energy, in the form of heat, must be effectively removed or rapid degradation of the discharge tube results. Additionally, excess heat in the gas discharge tube can have a negative effect on the accuracy of the analysis.
Several techniques have been used to address this problem. However, each technique has disadvantages. In one technique, the power is reduced resulting in a reduction of excess heat. This method has been found to be unsatisfactory because the sensitivity of the detector is reduced.
Another technique uses water as a coolant. The design of this detector is described in U.S. Pat. No. 4,654,504 entitled xe2x80x9cWater-Cooled Gas Discharge Detectorxe2x80x9d. Several disadvantages are associated with the use of water as a coolant. First, water restricts the materials that may be used to construct components of the detector that come in contact with the water. Typically, stainless steel parts are used to avoid rusting or other degradation of the material caused by contact with water. Stainless steel is difficult and expensive to machine compared to other materials such as copper, brass, or aluminum. Second, the water itself is an operational expense if not recirculated. Water is discarded at a rate of approximately one liter per minute. Alternatively, the water can be recirculated but a recirculating system is also expensive. Recirculating system requires a water pump capable of one liter per minute flow, a water reservoir, and a heat exchanger.
A significant disadvantage to using water as coolant is the inevitable down time of the detector. Many components in atypical gas flow system, such as gauges, valves, restrictors, and filters, can be damaged by exposure to water. Damage to these components may cause equipment failure requiring that replacement parts be installed. Additionally, water may contaminate the gas system leads requiring that the leads be allowed to bleed out until hydrogen and oxygen emissions drop. The detector cannot be in operation while these remedial measure are performed.
Finally, the use of water in an electrical instrument is generally undesirable due to the inevitable occurrence of electrical shorts and other problems.
A method for cooling the discharge tube in a gas discharge detector is described. This method uses air as the coolant fluid. The use of air minimizes or eliminates many disadvantages of previous cooling methods. An air cooled discharge detector is also disclosed.
In the method of the present invention, air is passed over the outside surface of the discharge tube thereby cooling the outside surface of the discharge tube, which causes cooling of its inside surface. The air can be supplied from various sources. Typical sources include central compressors commonly found in laboratories or on board air pumps. The only requirement of the air supply is that it must be capable of delivering a sufficient volume of air to cool the discharge tube.